The use of fixation and immobilization devices or templates for immobilizing a part of a body has become well known technology in applications such as orthotics and prosthetics, physical rehabilitation, and radiation oncology and diagnostic imaging. Those applications, in particular physical rehabilitation, require that the immobilization device is mouldable at activation temperature directly on a patient, that the immobilization device shows good mechanical properties and surface finishing and is light and comfortable to the patient. For radiation oncology it is desired that the device is transparent to the radiation and that the target part may be immobilized in a precise and reproducible position with respect to the irradiation source, thereby leaving limited possibility to the immobilized body part to move with respect to the irradiation source. In particular when high precision treatments—for example in Intensity Modulated Radiation Therapy, in Image Guided Radiation Therapy, in Stereotactic Radiation Therapy or Surgery—, or treatments with high energy, for example proton therapy, are involved where the target is very well defined and delineated, the immobilization device should permit precise, highly accurate and reproducible re-positioning of the anatomical area of interest, and assure a limitation of movement of less than 2 mm to ensure that the radiation is delivered to the target, at minimum risk to exposure of surrounding healthy tissue. Other applications for immobilization devices include physical rehabilitation applications and orthopedic applications for example in splints and braces, to immobilize and protect inflamed or injured joints, to support and immobilize ligaments and fractures and muscular structures and podiatry for example as insole (foot-bed) applications.
To produce fixation or immobilization devices, which are suitable for use in the above described applications, usually use is made of a sheet shaped thermoplastic material, which is molded to conform as good as possible to the body part that is to be immobilized. With high melting thermoplastics, a positive mould is used in the shape of the part that needs to be immobilized. Over the years, continuous development has been going on towards materials, which meet specific requirements of the envisaged application. The use of sheets of thermoplastic materials which may be directly moulded to the body part to be immobilized has achieved significant attention, as this permits achieving immobilization with the highest accuracy, where the size and shape of the immobilization device may be directly adapted to each individual patient in the position in which the body part is to be immobilized, and it may be adapted in the course of time by re-moulding the immobilization device. To permit this direct moulding, the thermoplastic material should have a melting temperature which is sufficiently low to be sustained by the body. Besides that the material should have sufficient formability and elasticity in the molten state, for a period of time which is sufficiently long to permit moulding, but not too long to save clinical time and minimize the risk to deformation after moulding has been completed. A thermoplastic material which is particularly suitable for direct moulding to a body part is ε-polycaprolactone. In practice, the ε-polycaprolactone sheet is heated in a warm water bath, for a period of time sufficiently long to achieve softening and melting of the material, the sheet is then applied to the body part to be immobilized, shaped to conform to the body part and allowed to cool.
Immobilization devices or templates made of these thermoplastic materials need to have a certain thickness to provide the required mechanical properties. In particular the material should have a bending modulus which is sufficiently high to minimize the risk to deformation of the device and the ability of the body part to move. The material should also provide a high stability during use of the immobilization device, stability meaning the restriction to move the body parts with respect to the immobilisation device or the radiation source, when immobilised by the immobilisation device. The higher the stability, the more difficult it will be to move the body parts in the immobilised condition. To improve comfort and permit evaporation of moisture through the material, the material has been perforated. The presence of perforations however reduces rigidity of the material and stability of the device.
Furthermore, thermoplastics shrink upon crystallization when cooling down from the melting temperature. This shrinking often results in a too tight fitting after cooling and crystallization, as the immobilization device is usually already moulded to fit closely to the body part that needs immobilization. In the course of the hours or days which follow moulding and crystallization, the thermoplastic material may show annealing, which often is associated with further shrinking. The result may be a device which fits too tightly, thus rendering the contact between the inner surface of the immobilization device and the patient uncomfortable. The degree of shrinking upon crystallization usually depends on the nature of the material, as well as on the geometrical design and size of the template.
To improve comfort to the patient, U.S. Pat. No. 3,957,262 discloses the use of a headpiece in the form of a cap which receives and supports the back of the patient's head that is not to be examined. The patient's head is restrained in a certain position within the cap by means of a chin restrainer and forehead restrainer. The interior of the headpiece is made of soft rubber foam which conforms to the size and shape of the back of the patient's head to be sustained by it. As a result, some movement within the headpiece will be possible and may be permitted, but accurate re-positioning cannot be guaranteed. No means are provided which permit adapting shape and dimensions of the cap to varying dimensions of the body part over time.
In another attempt to reduce discomfort provided by the immobilization device, the weight of the thermoplastic material has been reduced by reducing the density of the material, by using foamed thermoplastic materials. According to WO9611226 to (from?) Orfit Industries, incorporation of 2-6 wt. % of expanded polymer microspheres in epsilon-polycaprolactone, permitted reducing weight per volume unit in comparison with pure epsilon-polycaprolactone based thermoplastic sheets. The material appeared to be very well mouldable, showed good surface finishing and provided good comfort to the patient. However, loss of bending modulus to about 400 MPa had to be accepted.
In a further attempt to improve stiffness and stability, EP-A-1.582.187 discloses a hybrid mask with a first part which is intended to cover the body part to be immobilized. This first part is made of a thermoplastic material which takes the shape of a web or a net with a plurality of holes to allow evaporation of moisture from the skin. Along the edges of the first part, double material thickness is used to locally increase the bending modulus, with the purpose of improving the stability of the mask. The edges of the first part are connected to fasteners to permit mounting the mask to a positioning table for the patient. The thickness of the thermoplastic material varies between 1-5 mm, preferably between 2-4 mm depending on the envisaged rigidity, stiffness and stability, and on the porosity or number and size of the perforations.
Practical examples of immobilization devices are often made of a thermoplastic sheet with a thickness of 1.6-4.2 mm. When analyzing problems encountered with existing immobilization devices and thermoplastic materials used to produce those, it became clear that attempts to improve immobilisation resulted therein that sheet materials have been developed with reduced density, to permit increasing the thickness of the thermoplastic material to obtain a higher bending or flexural modulus and improve the stability, without giving in too much on comfort to the patient.
There is thus a need for an immobilisation device, which provides improved comfort to the patient, without however giving in on the mechanical properties. On the other hand, there is still a need for immobilization devices with improved mechanical properties.